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| Expansion of Gases |
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| Gases also expand on heating. Their volume expansion is very much greater than that of liquids. If a gas is heated at constant pressure, its volume increases and if a gas is heated at constant volume, its pressure increases due to expansion. Similarly, if we have to study the variation of pressure with temperature, its volume must be kept constant. |
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 A small gap is provided between the rails on railway tracks to make provision for the expansion of rails during summer, otherwise, the rails may bulge due to expansion causing train accidents. |
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 If a metal wire is used in a pendulum, its length changes with temperature. During summer, its length increases and thus, increasing its period and the pendulum clocks go slow in summer. During winter, the length decreases and hence, its period decreases. Thus, the pendulum clocks go fast during summer. |
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 In a cartwheel, the iron tyres are manufactured so that their diameter is slightly less than the wooden wheel on which they are fitted. The iron tyres are heated first. They expand and can be fitted easily into the wooden wheels. When cooled the iron wheels firmly grip the wooden wheels. |
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 Soda or soft drink bottles are made of thick glass. The reason for this is as follows-when a gas is heated in a closed container, its pressure increases. In a soda bottle this is exactly what happens, during summer heat, the soda bottles may explode due to pressure increase inside the bottle. Hence soda bottles are made of thick glass to withstand pressure increase. |
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| Generally, all liquids expand due to
heating. But water behaves in a peculiar way when heated from 0oC
to 4oC. On heating from 0oC up to 4oC its
volume decreases, reaches a minimum at 4oC such that the density
of water is maximum at 4oC on heating further, water behaves like
any other liquid. Similarly, during cooling process, the volume of water
decreases up to 4oC, reaches a minimum at 4oC, if
further cooled up to 0o C, its volume increases instead of
decreasing and hence, density of water at 0oC is lesser than that
at 4oC. Water freezes to ice at 0oC. Being less dense
than water, ice floats on water. Beyond 4oC, water behaves like
any other liquid. |
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| This unique feature of water between 0 and 40C, is a boon to aquatic animals. Let us see how- |
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| In cold countries and Polar Regions during winter, the temperature goes below 00C. In lakes and ponds when water at the surface reaches 40C, it is densest and goes to the bottom. When the temperature further reduces, only surface layers of water freeze whereas at the bottom water remains at 40C. Aquatic animals live comfortably in water at 40C ,whereas, outside temperatures may go much less than that. |
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